“If I knew someone was only going to stay six months, I would have never taken them on as a student.” Those exact words were spoken by a coach, describing a younger athlete who’d switched gyms at the end of a six-month contract.

Whether you empathize and are in complete agreement or whether you really can’t fathom sharing that perspective, you may not realize just how much generational differences can have an impact. When Millennials clash with coaches who...